1. Technical Field
The presently disclosed embodiments are directed to methods and systems for background suppression and noise suppression in binary to grayscale image conversion. More specifically, background inaccuracies produced in binary to grayscale image conversion are suppressed using an image enhancement function or tone reproduction curve, and noise created in transition areas during binary to grayscale image conversion is suppressed using a filter.
2. Description of Related Art
Conventionally, a typical black and white image on photographic film, for example, includes various gray levels of light. That is, different amounts of light are reflected from various spots of the image on the film, providing what is known as a continuous tone photographic image. It is conventionally known how to digitize the grayscale continuous tone photographic image. More specifically, each pixel or spot of the photographic image is assigned a number representing the amount of light or gray level of that particular spot. Typically, an eight-bit word is used, giving 256 different digitized gray levels of light. The digitized image is known as a continuous tone, or continuous tone, digital image. Further, it is possible to go back and forth between the analog and digital images and maintain a reasonable reproduction of the image.
It is also conventionally known to provide an image on a recording medium, for example, a paper sheet, rather than on photographic film. For example, a modulated laser can be used to scan a xerographic drum to give a series of black and white spots. The spots are formed by turning the laser on and off. The image on the drum is then developed and transferred to a copy sheet. This process of developing black and white spots provides a binary image, but does not generate a continuous tone image.
It is possible, however, to create the impression of a continuous tone image by using halftoning. The halftone process uses a mathematically stored screen pattern or array, for example, which is an almost-sinusoidal two-dimensional pattern. The process converts the original or continuous tone image into an image of black and white spots that “appears” to be a continuous tone image. This process is generally accomplished by systematically comparing each pixel's continuous tone value with the value of the screen. If the continuous tone value of the pixel is less dense than the screen value, then a white spot is produced. On the other hand, if the pixel value is more dense than the screen value, a black spot is produced. It should be understood that the pixel values are the 8-bit grayscale values for each pixel of the original image.
In effect, this procedure converts a grayscale image into black and white spots, but gives the impression of multiple gray levels by producing more white spots for a less-dense area and more black spots for a denser area. Although a true continuous tone image is not produced by this procedure, the procedure has two advantages. One advantage is that each spot of the image is described with one bit, rather than the eight-bit word used for each gray level pixel in the original continuous tone picture. This allows the halftone image to be stored with approximately ⅛ of the storage of the original continuous tone image. Another advantage is that, in fact, a halftone image can be printed on paper. In other words, the conversion takes each eight-bit pixel value representing a grayscale value, compares the pixel value to a screen value and provides either a zero (0) or a one (1) to modulate the laser. This image can then be printed on a recording medium such as paper.
The use of a tone reproduction curve (TRC) is described in U.S. Pat. No. 5,450,502 (hereinafter “the ‘502 patent’”), the disclosure of which is incorporated herein by reference. A TRC is defined as a function that describes the relationship of the input to the output within a system for the purposes of image enhancement. This function is then applied to the full input image.
Currently known techniques to convert binary images to continuous tone images are described in U.S. Pat. No. 6,343,159 (hereinafter “the ‘159 patent’”), the disclosure of which is incorporated herein by reference. These techniques include using a look up table (LUT) to convert one bit values in a neighbor of image pixels into grayscale values and using a spatial filter to approximate the original grayscale values.